In general, a hoist is used to convey goods in a warehouse and a railroad station and to disassemble and assemble machinery in a plant. The hoist is a kind of machinery, which serves to raise or lower a cargo using a wire and constitutes a group including, e.g., a motor, a reduction gear device and a wire drum, the hoist being provided with a hook at the tip of a wire thereof so as to lift the cargo.
The above described hoist includes a main frame, on which a plurality of parts is installed, the wire drum which is installed on the main frame and is provided at a circumferential surface thereof with a plurality of wire grooves to allow a wire to be wound on the wire drum, a winding sheave and a supporting sheave installed between the wire drum and the hook to guide the wire when the wire is unwound from the wire drum or is wound on the wire drum, a hoisting motor connected to the wire drum to rotate the wire drum, a decelerator installed between the hoisting motor and the wire drum to reduce revolutions per minute of the hoisting motor, a brake connected to the decelerator to control the decelerator, and a traveling unit installed to the main frame to transfer wheels installed to the bottom of the main frame along rails.
In such a conventional hoist, when the wire drum is rotated, the wire is wound on the wire grooves of the wire drum line by line, thereby acting to lift a load block and consequently, causing a load such as a cargo connected to the load block to be pulled up.
Here, since the wire is wound on the wire grooves formed in the surface of the wire drum line by line, the circumferential surface area of the wire drum should be increased in proportion to the rising and falling distance of the load block. Therefore, the longer the rising and falling distance of the load block is, the more increased the length or the diameter of the wire drum is in order to increase the surface area of the wire drum. To this end, a conventionally manufactured hoist requires for a wire drum having a length and a diameter suitable for the rising and falling distance of a load block and thus, there exists a need for the manufacture of various kinds of hoists according to a load lifting height.
Further, if the size of the wire drum is increased based on the increase of the rising and falling distance of the load block, sizes or volumes of related parts are increased that much. That is, the size of the main frame to install the larger wire drum thereon needs to be increased, and the powers of the decelerator and the hoisting motor to drive the larger wire drum need to be increased that much. Therefore, the conventional hoist has problems, such as the increase of the total size of the hoist according to the increase of the rising and falling distance of the load block, and the increase of power consumption.
Moreover, in the above described conventional hoist, the wire is wound on each wire groove of the wire drum line in a single line. Thus, when the wire is wound on or unwound from the wire drum, the wire tends to reciprocate from side to side along the wire groove within the width of the wire drum. As the wire reciprocates from side to side while being wound on or unwound from the wire drum, the wire may be frequently separated from the wire groove.
To solve the above described problem, the applicant of the present invention developed a hoist using friction wheels as disclosed in Korean Patent Application No. 10-2008-0090321.
The aforementioned conventional hoist using friction wheels includes: a pair of friction wheels connected to each other through a pinion, on outer circumferential surfaces of which is closely wound a wire in a single line; a wire drum on which one loose end of the wire at a drawing side is wound when the other end of the wire, connected to a load to be lifted, is pulled up by rotation of the friction wheels; an upper sheave to which the end of the wire at a winding side is secured such that a portion of the wire at the winding side, to which tension is applied by the load, passes through the upper sheave; and a lower sheave located below the upper sheave such that the wire secured to the upper sheave passes through the lower sheave to thereby be wound on the friction wheels, the lower sheave being raised or lowered along with the wire when the wire is wound on or unwound from the friction wheels.
In the above described conventional hoist using friction wheels, the wire at the winding side, to which the weight of the load to be lifted is directly applied, is connected from the lower sheave to the friction wheels through a narrow space between the upper sheave and the friction wheels.
As the wire at the winding side is connected from the lower sheave to the friction wheels through the narrow space, it is impossible to control a position of the lower sheave within a wide range. For example, when the lower sheave is horizontally moved to hang the load to be lifted on the hook, the wire at the winding side may come into contact with the bottom of the friction wheels or the upper sheave, which disadvantageously results in damage to parts in contact with the wire at the winding side to which a great force is applied, or damage to the wire due to unnecessary contact.
In conclusion, positioning the wire at the drawing side, which extends from the lower sheave to the friction wheels, in the narrow space considerably limits a horizontal movement range of the lower sheave and consequently, limits a lifting range of the load, resulting in considerable deterioration in lifting efficiency.
In addition, in the hoist using the conventional friction wheel, the wire at the drawing side may be wound on the wire drum in an unstable state thereof. That is, since the wire at the drawing side is sequentially wound on the circumference of the wire drum while being horizontally reciprocated in a longitudinal direction of the wire drum, the wire may not be stably guided immediately before being wound on the wire drum. This may prevent the wire from being sequentially wound on the circumference of the wire drum starting from one side to the other side of the wire drum and frequently causing the wire to deviate from a winding orbit thereof.
The conventional hoist using friction wheels also suffers from a complicated configuration of the friction wheels. Specifically, the two friction wheels are vertically arranged and the pinion is engaged between the pair of friction wheels to allow the friction wheels to be linked to each other. Thus, the configuration consisting of the two friction wheels and the pinion is somewhat complicated and production costs and assembly difficulty are increased accordingly.